DE102020007501A1 - Sensor element for gas sensor - Google Patents

Abstract

A sensor element includes an element base and a leading end protective layer, which is disposed around an outer periphery in a predetermined area including at least one end surface with a gas inlet, and is a porous layer including one or more unit layers, and has a thickness Tj ( j = 1 to n: n is a natural number) in µm of a j-th unit layer from one side of the element base on the end surface, a porosity pjin% of the j-th unit layer and a distance Lein mm from the heater to the end surface of the element base satisfy a predetermined inequality on an end surface of the leading end protective layer.

Description

BACKGROUND OF THE INVENTION

Field of invention

The present invention relates to a sensor element for a gas sensor and, in particular, to a surface protective layer therefor.

Description of the prior art

As a gas sensor for determining the concentration of a desired gas component contained in a measurement gas such as exhaust gas from an internal combustion engine, there is a gas sensor that includes a sensor element made of an oxygen ion conductive solid electrolyte such as zirconium dioxide (ZrO ₂ ) and some electrodes on the surface and has inside the same, is generally known. As a sensor element, a sensor element is known which has an elongated planar shape and comprises a protective layer formed from a porous body (porous protective layer) in an end portion in which a part for introducing the measurement gas is provided (see, for example, Japanese Patent Application Disclosure No. 2016-65852 ).

The protective layer is provided on the surface of the sensor element in order to ensure the water resistance of the sensor element when the gas sensor is in use. In particular, the protective layer is provided in order to prevent water-induced cracks of the sensor element under the action of a thermal shock in a case where water droplets adhere to the surface of the sensor element in a state of being heated by a heater inside the sensor element, caused by the heat (cold) of the water droplets on the sensor element.

In a case where the protective layer is provided on the planar sensor element as disclosed in Japanese Patent Application Laid-Open No. 2016-65852 disclosed, a portion of the protective layer farther from the heater tends to be more prone to thermal shock than a portion of the protective layer closer to the heater, and therefore water resistance may vary from location to location.

One possible measure is to increase the thickness of the protective layer in order to improve the resistance to thermal shock, but increasing the thickness leads to a decrease in the responsiveness and the temperature rise behavior of the sensor element. In particular, in a case where a gas inlet through which the measurement gas is introduced into the sensor element is provided in a leading end surface of the element, an excessive increase in the thickness of the protective layer covering the gas inlet is undesirable because it increases leads to a noticeable reduction in responsiveness.

SHORT REPRESENTATION

The present invention relates to a sensor element for a gas sensor and is directed in particular to a configuration of a surface protective layer of the same.

zu erfüllen,
wobei T_j (j = 1 bis n: n ist eine natürliche Zahl) eine Dicke in µm einer j-ten Einheitsschicht der Führungsende-Schutzschicht von einer Seite der Elementbasis auf der Endoberfläche ist, p_j eine Porosität in % der j-ten Einheitsschicht ist und L_e ein Abstand in mm von dem Heizer zur Endoberfläche ist.According to the present invention, a sensor element for a gas sensor includes: an element base that is a ceramic structure, the element base having an end surface with a gas inlet through which a measurement gas is introduced into the element base, and a sensor part for detecting a gas component to be measured and includes a heater for heating the sensor element; and a leading end protective layer disposed around an outer periphery of the element base in a predetermined area including at least the end surface and being a porous layer including one or more unit layers, the leading end protective layer being provided on the end surface, around $$\frac{1}{\mathrm{100,000}}{\displaystyle \sum \frac{{\text{T}}_{\text{j}}\cdot {\mathrm{\rho }}_{\text{j}}}{{\text{L.}}_{\text{e}}}>0.05}\left(\text{j}=1\sim \text{n}\right)$$

to meet
where T _j (j = 1 to n: n is a natural number) is a thickness in µm of a j-th unit layer of the leading end protective layer from one side of the element base on the end surface, p _{j is} a porosity in% of the j-th unit layer and L _{e is} a distance in mm from the heater to the end surface.

The thickness of a portion in which the gas inlet of the leading end protective layer is provided, which surrounds a portion of the element base in which the temperature becomes high when the gas sensor is in use, is determined by the porosity and the distance from the Heaters satisfy the predetermined inequality so that good water resistance of the portion of the sensor element can be ensured without causing a decrease in responsiveness.

The sensor element preferably has an elongated planar shape, and the end surface is preferably a surface on a side of a leading end portion in a longitudinal direction of the sensor element.

In this case, in the sensor element including the element base with the gas inlet at a leading end thereof, good water resistance of the leading end portion can be ensured without causing a decrease in responsiveness.

It is therefore an object of the present invention to provide a sensor element for a gas sensor which has a protective layer having a thickness corresponding to the desired water resistance on an end surface with a gas inlet.

Figure list

• 1 Fig. 3 is a schematic perspective external view of a sensor element 10 ;
• 2 Fig. 13 is a schematic view showing a configuration of a gas sensor 100 13, including a cross-sectional view taken along a longitudinal direction of the sensor element 10 has been recorded;
• 3rd shows the gas sensor 100 in a case where a leading end protective layer 2 a two-layer configuration of an inner lead end protective layer 2a and an outer lead end protective layer 2 B having;
• 4th Fig. 13 is a flowchart of processing in manufacturing the sensor element 10 ;
• 5 shows a case where the leading end protective layer 2 has a two-layer configuration and a non-uniform thickness;
• 6th Fig. 13 is a graph of a result of evaluation of water resistance according to Example 1 shown in Table 1 versus a leading end thickness index value; and
• 7th FIG. 13 is a graph of a result of evaluation of water resistance according to Example 2 shown in Table 2 versus leading end thickness index value along with the result of evaluation according to Example 1. FIG.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<Overview of Sensor Element and Gas Sensor>

1 Fig. 3 is a schematic perspective external view of a sensor element (gas sensor element) 10 according to an embodiment of the present invention. 2 Fig. 13 is a schematic view showing a configuration of a gas sensor 100 including a cross-sectional view along a longitudinal direction of the sensor element 10 shows. The sensor element 10 is a ceramic structure as the main component of the gas sensor 100 that detects a predetermined gas component in a measurement gas and measures its concentration. The sensor element 10 is a so-called limit current gas sensor element.

The gas sensor 100 includes next to the sensor element 10 essentially a pump cell power supply 30th , a heater power supply 40 and a controller 50 .

As in 1 shown, has the sensor element 10 has a configuration in which one end portion of an elongated planar element base 1 with a porous leading end protective layer 2 is covered.

As in 2 shown, includes the element base 1 an elongated, planar ceramic body 101 as the main structure; Main surface protective layers 170 are on two major surfaces of the ceramic body 101 provided and in the sensor element 10 is the leading end protective layer 2 further outside of both an end surface (a leading end surface 101e of the ceramic body 101 ) and four side surfaces are provided on one side of a leading end portion. The four side surfaces that are not opposite end surfaces in the longitudinal direction of the sensor element 10 (or the element base 1 or the ceramic body 101 ) are hereinafter simply referred to as side surfaces of the sensor element 10 (or the element base 1 or the ceramic body 101 ) designated.

The ceramic body 101 consists of ceramic, the main component of which is zirconium dioxide (yttrium-stabilized zirconium dioxide), which is a solid electrolyte that conducts oxygen ions. Various components of the sensor element 10 are outside and inside the ceramic body 101 intended. The ceramic body 101 with the configuration is tight and airtight. In the 2 configuration of the sensor element shown 10 is just an example and a specific configuration of the sensor element 10 is not limited to this configuration.

This in 2 shown sensor element 10 is a so-called serial gas sensor element with a three-chamber structure, which has a first inner chamber 102 , a second interior chamber 103 and a third interior chamber 104 inside the ceramic body 101 contains. That is, in the sensor element 10 is the first interior chamber 102 via a first diffusion control part 110 and a second diffusion control part 120 with a gas inlet 105 in connection, which extends on one side of an end section E1 of the ceramic body 101 (the element base 1 ) opens to the outside (to be precise, with the outside through the leading end protective layer 2 communicates), the second inner chamber 103 stands with the first interior chamber 102 via a third diffusion control part 130 in connection and the third interior chamber 104 stands with the second inner chamber 103 via a fourth diffusion control part 140 in connection. A path from the gas inlet 105 to the third inner chamber 104 is also referred to as the gas distribution part. In the sensor element 10 according to the present embodiment, the distributing part is straight along the longitudinal direction of the ceramic body 101 intended.

The first diffusion control part 110 , the second diffusion control part 120 , the third diffusion control part 130 and the fourth diffusion control part 140 are in 2 each provided as two vertically arranged slots. The first diffusion control part 110 , the second diffusion control part 120 , the third diffusion control part 130 and the fourth diffusion control part 140 offer the measuring gas flowing through them a predetermined diffusion resistance. Between the first diffusion control part 110 and the second diffusion control part 120 is a buffer space 115 provided, which has the effect of buffering the pulsation of the measuring gas.

An outer pumping electrode 141 is on an outer surface of the ceramic body 101 provided and an inner pump electrode 142 is in the first interior chamber 102 intended. There is also an auxiliary pump electrode 143 in the second inner chamber 103 provided and a measuring electrode 145 as a sensor part for the direct detection of a gas component to be measured is in the third inner chamber 104 intended. In addition, there is a reference gas inlet 106 , which communicates with the outside and through which a reference gas is introduced, on one side of the other end portion E2 of the ceramic body 101 provided and a reference electrode 147 is in the reference gas inlet 106 intended.

In a case where a goal of measuring the sensor element 10 For example, there is NOx in the measurement gas, the concentration of a NOx gas in the measurement gas is calculated by a method as described below.

First that is in the first inner chamber 102 Introduced measuring gas is set so that there is an essentially constant oxygen concentration by a pumping action (pumping in or pumping out oxygen) of a main pumping cell P1 has, and then into the second inner chamber 103 initiated. The main pumping cell P1 is an electrochemical pump cell with the outer pump electrode 141 , the inner pump electrode 142 and a ceramic layer 101a , which is part of the ceramic body located between these electrodes 101 is. In the second inner chamber 103 oxygen in the measurement gas is created by a pumping action of an auxiliary pump cell P2 , which is also an electrochemical pump cell, is pumped out of the element so that the measurement gas is in a sufficiently low oxygen partial pressure state. The auxiliary pumping cell P2 contains the outer pump electrode 141 , the auxiliary pumping electrode 143 and a ceramic layer 101b that is a section of the ceramic body 101 located between these electrodes.

The outer pumping electrode 141 , the inner pumping electrode 142 and the auxiliary pumping electrode 143 are each designed as a porous cermet electrode (eg a cermet electrode made of ZrO ₂ and Pt, the 1% Au contains). The inner pump electrode 142 and the auxiliary pumping electrode 143 that are to be in contact with the measurement gas are each formed from a material that has a weakened or no reducing power with respect to a NOx component in the measurement gas.

NOx in the sample gas that is passed through the auxiliary pump cell P2 is brought into the state of low oxygen partial pressure, is in the third inner chamber 104 initiated and through those in the third interior chamber 104 provided measuring electrode 145 reduced or decomposed. The measuring electrode 145 is a porous cermet electrode that also functions as a NOx reduction catalyst that is in an atmosphere in the third interior chamber 104 existing NOx reduced. During the reduction or decomposition there is a potential difference between the measuring electrode 145 and the reference electrode 147 kept constant. The oxygen ions generated by the aforementioned reduction or decomposition are passed through a metering pump cell P3 pumped out of the element. The measuring pump cell P3 contains the outer pump electrode 141 , the measuring electrode 145 and a ceramic layer 101c that is a section of the ceramic body 101 located between these electrodes. The measuring pump cell P3 is an electrochemical pump cell that pumps off oxygen produced by the decomposition of NOx in an atmosphere around the measuring electrode 145 is generated around.

The pumping (pumping in or out of oxygen) of the main pumping cell P1 , the auxiliary pumping cell P2 and the measuring pump cell P3 is under control of the controller 50 achieved by the fact that the pump cell power supply (variable power supply) 30th applies a voltage necessary for pumping across the electrodes contained in each of the pump cells. In the case of the measuring pump cell P3 becomes a voltage across the outer pump electrode 141 and the measuring electrode 145 applied so that the potential difference between the measuring electrode 145 and the reference electrode 147 is kept at a predetermined value. The pump cell power supply 30th is typically provided for each pump cell.

The controller 50 detects one between the measuring electrode 145 and the outer pumping electrode 141 flowing pump current Ip2 as a function of the flow through the measuring pump cell P3 pumped oxygen amount and calculates a NOx concentration in the measurement gas based on a linear relationship between a current value (NOx signal) of the pump current Ip2 and the concentration of decomposed NOx.

The gas sensor 100 preferably includes a plurality of electrochemical sensor cells, not shown, which measure the potential difference between each pump electrode and the reference electrode 147 capture, and each pumping cell is controlled by the controller 50 controlled based on a signal detected by each sensor cell.

In the sensor element 10 is a stoker 150 into the ceramic body 101 embedded. The stoker 150 is below the gas distribution part in 2 over an area in the vicinity of the one end portion E1 Provided up to at least one point where the measuring electrode 145 and the reference electrode 147 are formed. The stoker 150 is mainly intended for the sensor element 10 to heat up the oxygen ion conductivity of the solid electrolyte that makes up the ceramic body 101 forms to improve when the sensor element 10 is in use. In particular is the heater 150 so provided that it is covered by a layer of insulation 151 is surrounded.

The stoker 150 is a resistance heater, for example made of platinum. The stoker 150 generates heat by drawing from the heater power supply 40 under control of the controller 50 is powered.

The sensor element 10 according to the present embodiment is operated by the heater 150 heated so that the temperature in at least one area of the first inner chamber 102 to the second inner chamber 103 Is 500 ° C or more. In some cases the sensor element will 10 heated so that the temperature of the entire gas distribution part from the gas inlet 105 to the third inner chamber 104 Is 500 ° C or more. This is done to increase the oxygen ion conductivity of the solid electrolyte that forms each pump cell and to demonstrate the capability of each pump cell. In this case, the temperature becomes close to the first inner chamber 102 that becomes the highest temperature, about 700 ° C to 800 ° C.

In the following description of the two main surfaces of the element base 1 (the ceramic body 101 ) a major surface (or an outer surface of the sensor element 10 with the main surface), which is located on an upper side in 2 and on one side on which the main pumping cell P1 , the auxiliary pumping cell P2 and the measuring pump cell P3 are mainly provided, also referred to as a pumping surface Ip, and a main surface (or an outer surface of the sensor element 10 with the main surface), which is on a lower side in 2 and on one side, on which the heater 150 is provided, is also called a heater surface 1h designated. In other words, the pumping surface Ip is a major surface closer to the gas inlet 105 , the three inner chambers and the pump cells than on the heater 150 and the heater surface 1h is a major surface that is closer to the heater 150 than at the gas inlet 105 , the three inner chambers and the pump cells.

A variety of electrode connections 160 are on the respective main surfaces of the ceramic body 101 on the side of the other end portion E2 designed to establish an electrical connection between the sensor element 10 and the outside. These electrode connections 160 are with the aforementioned five electrodes, the opposite ends of the heater 150 and a line for detecting the heater resistance, not shown, via lines made inside the ceramic body 101 are provided, electrically connected to establish a predetermined correspondence relationship. The application of a voltage from the pump cell power supply 30th to each pump cell of the sensor element 10 and the heating by the heater 150 from the heater power supply 40 is fed, thus take place via the electrode connections 160 .

The sensor element 10 also contains the above-mentioned main surface protective layers 170 ( 170a and 170b ) on the pump surface 1p and the heater surface 1h of the ceramic body 101 . The main surface protective layers 170 are layers of aluminum oxide that have a thickness of about 5 µm to 30 µm and contain pores with a porosity of about 20% to 40% and are intended to prevent foreign bodies and toxic substances from adhering to the main surfaces (the pump surface 1p and the heater surface 1h ) of the ceramic body 101 and the outer pumping electrode 141 that are on one side of the pumping surface 1p is intended to prevent. The main surface protective layer 170a on the side of the pumping surface 1p thus acts as a protective pumping electrode layer to protect the outer pumping electrode 141 .

In the present embodiment, the porosity is determined by applying a known image processing method (for example, binarization processing) to a scanning electron microscope (SEM) image of an evaluation target.

The main surface protective layers 170 are essentially over the entire pump surface 1p and the heater surface 1h provided, with the exception of those in 2 partially exposed electrode connections 160 but this is just an example. The main surface protective layers 170 compared to the in 2 case shown locally in the vicinity of the outer pump electrode 141 on the side of one end portion E1 be provided.

<Details of the leading end protective layer>

In the sensor element 10 is the leading end protective layer 2 around an outermost periphery of the element base 1 provided around having a configuration as in a predetermined range from the one end portion E1 having.

The leading end protective layer 2 is designed to cover a portion of the element base 1 where the temperature becomes high (up to about 700 ° C to 800 ° C) when the gas sensor 100 is in use to ensure water resistance in the portion, thereby preventing cracks (water-induced cracks) from occurring in the element base 1 due to thermal shock caused by a local temperature decrease when the section is in direct contact with water.

In the present embodiment, as a result of repeatedly dripping a predetermined amount of water on the leading end protective layer 2 until an abnormality of the pumping current Ip0 occurs before and after the dropping, a maximum amount of the dropped water that does not cause the abnormality of the pumping current Ip0 is defined as the critical amount of water. Whether or not the water resistance is good is determined from the magnitude of a value of the critical amount of water. In this case, the term “water resistance” can be used in the sense of the critical amount of water.

In addition, there is the leading end protective layer 2 intended to ensure resistance to poisoning, to prevent poisoning substances such as Mg from penetrating into the sensor element 10 to prevent.

The leading end protective layer 2 is to cover the leading end surface 101e and the four side surfaces of the element base 1 on the side of one end portion E1 (around an outer periphery of the element base 1 on the side of one end portion E1 ) intended. A section of the Leading end protective layer 2 on one side of the leading end surface 101e is used in particular as a leading section 2e denotes a portion of the leading end protective layer 2 on the side of the pumping surface 1p is used in particular as a pumping surface section 2p and a portion of the leading end protective layer 2 on one side of the heater surface 1h is used in particular as a heater surface section 2h designated.

The leading end protective layer 2 made of alumina has a porosity of 10% to 40%. The leading end protective layer 2 is provided as a layer with low thermal conductivity with high porosity in order to have a function of heat conduction from the outside to the element carrier 1 to suppress.

The leading end protective layer 2 is designed so that it has a thickness which results from the positional relationship between the two main surfaces (the pump surface 1p and the heater surface 1h ) the element base 1 as the main educational target areas, the leading end surface 101e of the ceramic body 101 with the gas inlet 105 and the stoker 150 results while the water resistance of the sensor element 10 is ensured. The details will be described later.

The leading end protective layer 2 is made by sequential thermal spraying (plasma spraying) of an appropriate material on the element base 1 educated. This creates an anchoring effect between the element base 1 and the leading end protective layer 2 Developed to thereby ensure a secure bonding (adhesion) of the leading end protective layer 2 with the element base 1 to guarantee.

<Case where the leading end protective layer has a laminated structure>

While in 2 the sensor element 10 including the leading end protective layer 2 shown as a single layer, the leading end protective layer 2 a laminate structure in which two or more layers (unit layers) are laminated.

3rd shows the gas sensor 100 in a case where the leading end protective layer 2 a two-layer configuration of an inner lead end protective layer 2a and an outer lead end protective layer 2 B having. In an in 3rd illustrated case is the inner leading end protective layer 2a the same layer as the leading end protective layer 2 of the in 2 shown sensor element 10 and the outer leading end protective layer 2 B is provided around the inner lead end protective layer 2a to surround.

The outer leading end protective layer 2 B is made of alumina and has a porosity of 10% to 40% lower than any other layer (the inner lead end protective layer 2a in the in 3rd case shown), which is located within the outer leading end protective layer 2 B is located on. In the 3rd shown leading end protective layer 2 thus has a configuration in which a layer having a lower thermal conductivity than the outer leading end protective layer 2 B with the outer leading end protective layer 2 B is covered, which has a lower porosity than the layer.

As with the inner leading end protective layer 2a becomes the outer leading end protective layer 2 B formed by sequential thermal spraying (plasma spraying) of a material therefor.

In a case where the leading end protective layer 2 having the laminated structure described above, each unit layer is formed to have a thickness in view of the positional relationship between the pumping surface 1p , the heater surface 1h , the leading surface 101e and the stoker 150 while maintaining the water resistance of the sensor element 10 ensures. Consequently, the leading end protective layer has 2 a total thickness in terms of positional relationship.

Between the element base 1 and the inner leading end protective layer 2a For example, an undercoat, not shown, may be formed to adhere the inner lead end protective layer 2a to improve. In contrast to the inner leading end protective layer 2a and the like made by thermal spraying after the element base is completed 1 is formed, the underlayer is formed simultaneously with the element base 1 educated.

<Thickness of each portion of the leading end protective layer>

The thickness (the total thickness) of each portion of the leading end protective layer 2 that are in the sensor element 10 of the gas sensor 100 according to the present embodiment is included next described. In the present embodiment, the leading end protective layer is 2 formed to have a thickness resulting from the positional relationship between the pumping surface 1p , the heater surface 1h , the leading surface 101e and the stoker 150 results as described above.

First, a relationship between the thickness of the leading end protective layer will be discussed 2 on each side of the pumping surface 1p and the side of the heater surface 1h and the position of the heater 150 described.

The leading end protective layer 2 is provided so that the following inequality (1) both in the pumping surface portion 2p as well as in the heater surface section 2h is satisfied, where n (n is a natural number) is the number of unit layers that make up the leading end protective layer 2 form, T _{s, i} (i = 1 to n) the thickness in µm of the i-th unit layer from one side of the element base 1 is (s = p in the case of the thickness of the pumping surface portion 2p , and s = h in the case of the thickness of the heater surface portion 2h) , p _{i is} the porosity in% of the i-th unit layer and L _{s is} the distance in mm from the heater 150 to the main surface of the element base 1 (s = p in the case of the distance on the side of the pumping surface 1p and s = h in the case of the clearance on the side of the heater surface 1h ) is. $$\frac{1}{\mathrm{100,000}}{\displaystyle \sum \frac{{\text{T}}_{\text{s}\text{, i}}\cdot {\rho }_{\text{i}}}{{\text{L.}}_{\text{e}}}>0.1}\left(\text{s}=\text{p or h; i}=1\sim \text{n}\right)$$

In this case there is good water resistance of the pump surface portion 2p and the heater surface portion 2h guaranteed. In particular, a water resistance of more than 6 µl can be achieved. A value on the left side of the inequality is hereinafter referred to as a major surface thickness index value.

In a case where the number of unit layers is one (i = 1), as in the one in FIG 2 shown sensor element 10 , a subscript i can be omitted from inequality (1). In 2 is T _{p, i} = T _p and T _{h, i} = T _h . In 3rd is T _{p, 1} = T _p1 , T _{h, 1} = T _h1 , T _{p, 2} = T _p2 and T _{h, 2} = T _h2 .

It is known that the major surface thickness index value has a positive correlation with water resistance. That is, the sensor element 10 having a larger major surface thickness index value indicates better water resistance on either side of the pumping surface 1p and the side of the heater surface 1h of the sensor element 10 on. In particular, a term T _{s, i} · pi / L _s that provides the major surface thickness index value is inversely proportional to the distance L _s from the heater 150 to the main surface of the element base 1 and is proportional to the thickness T _{s, i of} the unit layer. Inequality (1) thus means that the pump surface portion is excellent in water resistance 2p and the heater surface portion 2h can be obtained by adding the unit layers that make up the pumping surface portion 2p and the unit layers that form the heater surface portion 2h form, each on the side of the pump surface 1p and the side of the heater surface 1h may be provided so that each unit layer has a thickness considering the distance L _s from the heater 150 to the main surface of the element base 1 having. This is presumably because the greater the distance between the heater 150 and each major surface of the element base 1 a temperature difference inside the sensor element 10 increases and the thermal shock resistance is deteriorated. The one in the 2 and 3rd shown sensor element 10 is the heater surface 1h closer to the heater 150 than the pumping surface 1p so that the heater surface portion 2h with a smaller thickness than the pumping surface portion 2p is configured to cause the heater surface portion 2h and the pumping surface portion 2p have an equivalent water resistance.

On the other hand, there is an excessive increase in the thickness of the inner leading end protective layer 2a and the outer leading end protective layer 2 B in each of the pumping surface sections 2p and the heater surface portion 2h not preferred because of thermal stress on the heater 150 when the temperature rises, increases, and as a result, the sensor element 10 tear. From this point of view, the inner leading end protective layer 2a preferably a thickness of 800 µm or less and the outer leading end protective layer 2 B preferably has a thickness of 400 µm or less in each of the pump surface portion 2p and the heater surface portion 2h on.

A relationship between the thickness of the leading end protective layer 2 on one side of the leading end surface 101e with the gas inlet 105 and the position of the heater 150 will be described next.

The leading end protective layer 2 is on the leader surface 101e provided so that an inequality (2) below is satisfied, where n (n is a natural number) is the number of unit layers that make up the leading end portion 2e the leading end protective layer 2 form, T _j (j = 1 to n) the thickness (a size in the lengthwise direction of the element) of the j-th unit layer from the side of the element body 1 is, p _{j is} the porosity in% of the j-th unit layer and L _{e is} the distance in mm from the heater 150 to the leading end surface 101e is. $$\frac{1}{\mathrm{100,000}}{\displaystyle \sum \frac{{\text{T}}_{\text{j}}\cdot {\mathrm{\rho }}_{\text{j}}}{{\text{L.}}_{\text{e}}}>0.05}\left(\text{j}=1\sim \text{n}\right)$$

In this case, the water resistance of the leading end portion becomes good 2e the leading end protective layer 2 ensured. In particular, a water resistance of more than 5 μl can be obtained. A value on a left side of the inequality is hereinafter referred to as a leading end thickness index value.

• 300 µm ≤ T₁ ≤ 500 µm;
• 20% ≤ p₁ ≤ 30%; und
• 0,35 mm ≤ L_e ≤ 1,3 mm.

In a single layer case, as in 2 is shown, the leading end protective layer 2 which satisfies inequality (2) is achieved, for example, when:

• 300 µm T ₁ 500 µm;
• 20% ≤ p ₁ ≤ 30%; and
• 0.35 mm ≤ L _e ≤ 1.3 mm.

• 300 µm ≤ T₁ ≤ 850 µm;
• 40% ≤ p_i ≤ 80%;
• 150 µm≤ T₂ ≤ 350 µm;
• 15% ≤ p₂ ≤ 40%; und
• 0,35 mm ≤ L_e ≤ 1,3 mm.

In a case of the two-tier configuration as shown in 3rd is shown, the leading end protective layer 2 which satisfies inequality (2) is achieved, for example, when:

• 300 µm T ₁ 850 µm;
• 40% ≤ p _i ≤ 80%;
• 150 µm T ₂ 350 µm;
• 15% ≤ p ₂ ≤ 40%; and
• 0.35 mm ≤ L _e ≤ 1.3 mm.

As with the major surface thickness index value, the leading end thickness index value is known to have a positive correlation with water resistance. That is, the sensor element 10 with a larger leading end thickness index value, better water resistance on the one end portion side E1 having. In particular, a term T _j * p _j / L _e that provides the leading end thickness index value is inversely proportional to the distance L _e from the heater 150 to the leading end surface 101e and is proportional to the thickness T _{j of} the unit layer on the side of the one end portion E1 . Thus, inequality (2) means that the leading end portion is excellent in water resistance 2e can be achieved in that the unit layers that make up the leading end portion 2e should be provided so that each unit layer has a thickness considering the distance L _e from the heater 150 to the leading end surface 101e having. This is presumably also because the greater the distance between the heater 150 and the leading end surface 101e the temperature difference inside the sensor element 10 increases and the thermal shock resistance is deteriorated.

Viewed differently, the water resistance of the sensor element is 10 secured as long as the thickness of each unit layer is determined so that the inequality (1) in each pumping surface portion 2p and the heater surface portion 2h is satisfied, and inequality (2) in the leading section 2e is satisfied. So it can be said that there is little need for the leading end protective layer 2 with an extremely large thickness. In such a case, the decrease in responsiveness and temperature rise behavior becomes a problem. Thus, it can be said that inequalities (1) and (2) are requirements to ensure water resistance on the main surface side and on the one end portion side without causing decrease in responsiveness and temperature rise behavior.

The leading end protective layer 2 is preferably provided to have the two-layer configuration of the inner lead end protective layer 2a and the outer leading end protective layer 2 B as in 3rd and that it satisfies an inequality (3) below. $$\frac{1}{\mathrm{100,000}}{\displaystyle \sum \frac{{\text{T}}_{\text{j}}\cdot {\mathrm{\rho }}_{\text{j}}}{{\text{L.}}_{\text{e}}}>0.18}\left(\text{j}=1.2\right)$$

In this case, the leading end portion becomes better water resistance 2e ensured. In particular, a water resistance of more than 10 μl can be obtained.

• 300 µm ≤ T₁ ≤ 850 µm;
• 40% ≤ p_i ≤ 80%;
• 150 µm ≤ T₂ ≤ 350 µm;
• 15% ≤ p₂ ≤ 40%; und
• 0,35 mm ≤ L_e ≤ 1,3 mm.

The leading end protective layer 2 which satisfies inequality (3) is reached, for example, when:

• 300 µm T ₁ 850 µm;
• 40% ≤ p _i ≤ 80%;
• 150 µm T ₂ 350 µm;
• 15% ≤ p ₂ ≤ 40%; and
• 0.35 mm ≤ L _e ≤ 1.3 mm.

The leading end protective layer 2 is preferably on the leading end surface 101e is provided to satisfy an inequality (4) below. $$\frac{1}{\mathrm{100,000}}{\displaystyle \sum \frac{{\text{T}}_{\text{j}}\cdot {\mathrm{\rho }}_{\text{j}}}{{\text{L.}}_{\text{e}}}>0.25}\left(\text{j}=1.2\right)$$

In this case, the water resistance of the leading end portion becomes extremely good 2e ensured. In particular, a water resistance of more than 20 μl can be obtained.

• 300 µm ≤ T₁ ≤ 850 µm;
• 50% ≤ p_i ≤ 80%;
• 250 µm ≤ T₂ ≤ 350 µm;
• 15% ≤ p₂ ≤ 40%; und
• 0,35 mm ≤ L_e ≤ 1,3 mm.

The leading end protective layer 2 which satisfies inequality (4) is reached, for example, when:

• 300 µm T ₁ 850 µm;
• 50% ≤ p _i ≤ 80%;
• 250 µm T ₂ 350 µm;
• 15% ≤ p ₂ ≤ 40%; and
• 0.35 mm ≤ L _e ≤ 1.3 mm.

As stated above, in the sensor element according to the present embodiment, the thickness of a portion in which the gas inlet of the leading end protective layer is provided surrounding a portion of the element base where the temperature becomes high when the gas sensor is in use is as follows is determined to satisfy Inequality (2), so that good water resistance of the portion of the sensor element can be ensured without causing a decrease in responsiveness.

<Method of manufacturing the sensor element>

An example of a method for manufacturing the sensor element 10 having a configuration and features as described above will next be described. 4th Fig. 13 is a flowchart of processing in manufacturing the sensor element 10 , taking as an example a case where the leading end protective layer 2 the inner leading end protective layer 2a and the outer leading end protective layer 2 B , as in 3rd shown includes.

When making the element base 1 First, a plurality of green sheets (not shown) which are green sheets containing the oxygen ion conductive solid electrolyte such as zirconia as a ceramic component and on which no pattern is formed are prepared (step S1 ).

The raw panels have a large number of panel holes that are used for positioning during printing and lamination. The plate holes are machined into the raw plates before the pattern is produced, for example by punching with a punching machine. Green sheets representing a portion of the ceramic body 101 in which an inner space is formed also include penetrating portions corresponding to the inner space formed in advance, for example, by punching as described above has been. The raw panels need not be of the same thickness and can have different thicknesses in accordance with the corresponding sections of the element base ultimately formed 1 exhibit.

After the production of the raw panels corresponding to the respective layers, pattern printing and drying are carried out on the individual raw panels (step S2 ). Specifically, a pattern of various electrodes becomes a pattern of the heater 150 and the insulating layer 151 , a pattern of the electrode connections 160 , a sample of the main surface protective layers 170 , an internal wiring pattern, not shown, and the like are formed. Applying or placing a sublimable material (escape material) to form the first diffusion control member 110 , the second diffusion control part 120 , the third diffusion control part 130 and the fourth diffusion control part 140 is also carried out at the time the sample is printed. In a case where the underlayer is formed, a pattern for forming the underlayer is printed on green sheets which, after lamination, become an uppermost layer and a lowermost layer.

The patterns are printed on the raw panels by applying pastes for pattern formation in accordance with the properties required for the respective educational objectives using known screen printing technology. A known drying device can be used for drying after printing.

After printing patterns on the individual raw panels, a bonding paste is printed and dried to laminate and bond the green panels (step S3 ). The known screen printing technique can be used for printing the bonding paste and the known drying device can be used for drying after printing.

The green sheets to which an adhesive has been applied are then stacked in a predetermined order, and the stacked green sheets are crimped under predetermined temperature and pressure conditions to thereby form a laminated body (step S4 ). Specifically, the crimping is performed by stacking and holding the green sheets as the target of lamination on a predetermined laminating device, not shown, while positioning the green sheets on the plate holes, and then placing the green sheets together with the laminating device using a laminating machine, such as a well-known hydraulic press machine, heated and pressurized. The pressure, temperature and time for heating and pressurization depend on the laminating machine to be used, and these conditions can be determined accordingly to achieve good lamination. The pattern for forming the underlayer can be formed on the laminated body thus obtained.

After the laminated body is obtained as described above, the laminated body is cut out at a plurality of places to obtain unit bodies which ultimately become the individual element bases 1 be (step S5 ). The unit bodies obtained in this way are then each fired at a firing temperature of approximately 1300 ° C. to 1500 ° C. (step S6 ). This becomes the element base 1 produced. That is, the element base 1 is made by integral firing of the ceramic body 101 from the solid electrolyte, the electrodes and the main surface protective layers 170 generated. The integral firing is carried out in this way, so that the electrodes each have sufficient adhesive strength in the element base 1 exhibit.

After the element base 1 prepared in the above manner, the inner leading end protective layer 2a and the outer leading end protective layer 2 B in terms of the element base 1 educated. The inner leading end protective layer 2a is formed by thermally spraying powder (alumina powder) to form the inner leading end protective layer, which was previously applied to a location on the element base 1 as the target of forming the inner leading end protective layer 2a was made to have an intended thickness (step S7 ) and then firing the element base 1 on which an applied film has been formed in the above manner (step S8 ). The alumina powder for forming the inner leading end protective layer contains alumina powder having a predetermined particle size distribution and a pore-forming material in a ratio corresponding to a desired porosity, and a pore-forming material is made by firing the element base 1 pyrolyzed after thermal spraying to suitably form the inner lead end protective layer 2a with a high porosity of 40% to 80%. The known technology can be used for thermal spraying and baking.

The inner leading end protective layer 2a can be made to have different thicknesses on the side of the pumping surface 1p , on the side of the heater surface 1h and on the leading end surface side 101e to have, for example, by reducing the speed of the thermal Spraying, repeating thermal spraying on the same portion, and other methods in forming on a side where the thickness is to be increased.

After the formation of the inner leading end protective layer 2a For example, powder (alumina powder) for forming the outer leading end protective layer, which has also been prepared in advance and contains alumina powder having a predetermined particle size distribution, is placed in one position of the element base 1 as the target of forming the outer leading end protective layer 2 B thermally sprayed to have an intended thickness (step S9 ) to thereby create the outer leading end protective layer 2 B to form with a desired porosity. The alumina powder for forming the outer leading end protective layer does not contain the pore-forming material. The known technique is also applicable to thermal spraying. The measures to be taken in a case where the outer leading end protective layer 2 B on the side of the pumping surface 1p , on the side of the heater surface 1h and on the leading end surface side 101e having different thicknesses are similar to those used in the formation of the inner lead end protective layer 2a be seized.

In a case where the leading end protective layer 2 provided as a single layer, as in 2 is the above-mentioned step S9 unnecessary.

The sensor element 10 is obtained by the above-mentioned methods. The sensor element thus obtained 10 is housed in a predetermined housing and in the body (not shown) of the gas sensor 100 built-in.

<modifications>

The embodiment described above is directed to a sensor element having three inner chambers, but the sensor element need not necessarily have a three-chamber structure. That is, the sensor element can have an inner chamber or two inner chambers.

The above-mentioned embodiment is based on the assumption that the unit layers constituting the leading end protective layer 2 form a uniform thickness in the leading end portion 2e but the thickness in the leading end portion 2e depends on the shape of the leading end protective layer 2 , sometimes not uniform. An example of such a case is a case where the thickness T _{p is} on the side of the pumping surface 1p or the thickness T _h on the side of the heater surface 1h is intentionally made to differ from the thickness T _e on the leading end surface side 101e to distinguish.

und $${\text{T}}_2=\left({\text{T}}_{\text{2p}}+{\text{T}}_{\text{2h}}+{\text{T}}_{\text{2c}}\right)/3.$$

5 shows a case where the leading end protective layer 2 has a two-layer configuration and has an uneven thickness. In such a case, after the size can be along the pumping surface 1p the element base 1 , the size along the heater surface 1h the element base 1 and the size on one of the pumping surface 1p and the heater surface 1h equidistant position have been specified, an average value of them can be used as a value of T _j in the aforementioned inequalities (2) to (4). In an in 5 In the illustrated case, the following equations are fulfilled: $${\text{T}}_{\text{1}}=\left({\text{T}}_{\text{1p}}+{\text{T}}_{\text{1h}}+{\text{T}}_{\text{1e}}\right)/\mathrm{3rd};$$

and $${\text{<figure-callout id="2" label="T" filenames="DE102020007501A1\_0011.png,DE102020007501A1\_0012.png" state="{{state}}">T</figure-callout>}}_2=\left({\text{<figure-callout id="2p" label="T" filenames="DE102020007501A1\_0012.png" state="{{state}}">T</figure-callout>}}_{\text{2p}}+{\text{<figure-callout id="2h" label="T" filenames="DE102020007501A1\_0012.png,DE102020007501A1\_0013.png" state="{{state}}">T</figure-callout>}}_{\text{2h}}+{\text{T}}_{\text{2c}}\right)/\mathrm{3rd}.$$

In the above-mentioned embodiment, after the powder is thermally sprayed to form the inner leading end protective layer in step S7 the burning in the crotch S8 and then thermal spraying of the powder for forming the outer leading end protective layer is carried out in the step S9 carried out. Burning in crotch S8 and thermal spraying in step S9 however, they can also be carried out in reverse order.

In the above-mentioned embodiment, the inner leading end protective layer is made 2a and the outer leading end protective layer 2 B each of alumina, and the alumina powder is used as a material for thermal spraying in forming the two layers, but they do not necessarily have to be made of alumina. The inner leading end protective layer 2a and the outer Leading end protective layer 2 B Instead of aluminum oxide, metallic oxide such as zirconium dioxide (ZrO ₂ ), spinel (MgAl ₂ O ₄ ) and mullite (Al ₆ O ₁₃ Si ₂ ) can be produced. In this case, the powder of the metal oxide can be used as a material for thermal spraying.

The aforementioned embodiment is based on the sensor element 10 aligned, which is the elongated planar element base 1 (the ceramic body 101 ) with the gas inlet 105 in one end section E1 thereof, but even in a case where the gas inlet 105 in a side portion of the sensor element 1 is arranged, effects similar to those obtained in the aforementioned embodiment are expected as long as inequalities (2) to (4) are satisfied.

[Examples]

(Example 1)

As Example 1, ten types of sensor elements were used 10 with leading end protective layers 2 which are each a single layer and have different thicknesses T ₁ in the leading end portion 2e have, and the water resistance in the leading end portion 2e each of the sensor elements 10 was rated.

In particular, two types of element bases have been identified 1 , including an element base 1 with L _p of 0.91 mm, L _h of 0.41 mm and L _e of 1.26 mm (hereinafter referred to as base sample α) and an element base 1 with L _p of 1.03 mm, L _h of 0.20 mm and L _e of 0.38 mm (hereinafter referred to as base sample b).

As for the leading end protective layers 2 of the base sample is concerned with α, the thickness T ₁ in the leading end portion became 2e varied in seven steps of 50 µm, 100 µm, 150 µm, 200 µm, 300 µm, 400 µm and 500 µm and the porosity p ₁ was varied in three steps of 20%, 25% and 30%.

The leading end protective layer 2 of the base sample b had a thickness T ₁ in the leading end portion 2e of 380 µm and a porosity p ₁ of 22%.

The water resistance was evaluated by placing a 0.1 µl water drop on the side of one end portion E1 of the sensor element 10 was applied while the pumping current Ip0 through the main pumping cell P1 was measured in a state that each of the sensor elements 10 by the heater 150 was heated to about 500 ° C to 900 ° C, and by specifying the maximum amount of water that does not cause an abnormality in a measurement output.

The distance L _e in each of the sensor elements 10 , the thickness T ₁ and the porosity p ₁ in the leading end portion 2e the leading end protective layer 2 , the leading end thickness index value calculated therefrom and the result of the evaluation of the water resistance are shown in Table 1 as a list. 6th Fig. 13 is a graph of the result of evaluation of water resistance according to Example 1 shown in Table 1 against the leading end thickness index value. [Table 1] L _e LEADING END PROTECTIVE LAYER LEADING END THICKNESS INDEX VALUE WATER RESISTANCE [mm] T ₁ [µm] ρ1 [%] [µl] 1.26 400 25th 0.079 6.8 1.26 100 25th 0.020 4.4 1.26 300 25th 0.060 5.7 1.26 50 25th 0.010 4.7 1.26 200 25th 0.040 5 1.26 150 25th 0.030 4.9 0.38 380 22nd 0.224 16.0 1.26 500 25th 0.099 7.8 - 1.26 400 20th 0.063 5.8 1.26 300 30th 0.071 6.8

As shown in Table 1, each of the sensor elements had 10 whose leading end thickness index value satisfied Inequality (2) had water resistance of 5 µl or more.

It's over 6th It can be seen that the leading end thickness index value has a positive correlation with water resistance regardless of the element base size difference 1 , and it will look out too 6th confirms that water resistance greater than 5 µl can be achieved when the leading end thickness index value exceeds 0.05. This indicates that the water resistance is in the one end section E1 of the sensor element 10 , in which the gas inlet 105 is provided is appropriately secured as long as the leading end protective layer 2 is provided so that the leading end thickness index value satisfies inequality (2).

(Example 2)

As Example 2, there were 12 types of sensor elements 10 each of the leading end protective layer 2 with the two-layer configuration of the inner leading end protective layer 2a and the outer leading end protective layer 2 B included, manufactured, and the water resistance in the leading end section 2e each of the sensor elements 10 was rated.

In particular, two types of element bases have been identified 1 including the base sample α as in Example 1 and an element base 1 with L _p of 0.71 mm, L _h of 0.17 mm and L _e of 0.39 mm (hereinafter a base sample c).

As for the inner leading end protective layers 2a of the base sample a, the thickness became T ₁ in the leading end portion 2e varied in seven steps of 350 µm, 400 µm, 500 µm, 600 µm, 650 µm, 800 µm and 850 µm and the porosity p ₁ was varied in seven steps of 35%, 40%, 50%, 55%, 60%, 65% and 80% varied.

On the other hand, as with the outer leading end protective layers 2 B , the thickness T ₂ in the leading end portion 2e varied in three steps of 150 μm, 250 μm and 350 μm and the porosity p ₂ was varied in three steps of 15%, 20% and 25%.

The inner leading end protective layer 2a of the base sample c had a thickness T ₁ in the leading end portion 2e of 276 µm and a porosity p ₁ of 16.2%. In addition, the outer leading end protective layer had 2 B a thickness T ₂ in the leading end portion 2e of 232 µm and a porosity p ₂ of 40.7%.

The distance L _e on the leading end surface side 101e , the thickness T ₁ and the porosity p _{1 of} the inner leading end protective layer 2a (“FIRST PROTECTIVE LAYER” in Table 2), the thickness T ₂ and the porosity p _{2 of} the outer leading end protective layer 2 B (“SECOND PROTECTIVE LAYER” in Table 2), the leading end thickness index value calculated therefrom and the result of the evaluation of the water resistance are shown in Table 2 as a list. [Table 2] L _e FIRST PROTECTIVE LAYER SECOND PROTECTIVE LAYER LEADING END THICKNESS INDEX VALUE WATER RESISTANCE [mm] T ₁ [µm] p ₁ [%] T ₂ [µm] p ₂ [%] [µl] 1.26 400 50 250 25th 0.208 14th 1.26 650 40 250 25th 0.256 20th 1.26 400 40 250 25th 0.177 11.2 1.26 500 40 250 25th 0.208 16.3 0.39 276 16.2 232 40.7 0.357 20.4 1.26 800 55 250 25th 0.399 43.2 1.26 800 35 250 25th 0.272 30.9 1.26 800 80 250 25th 0.558 20th 1.26 600 60 350 25th 0.355 25th 1.26 850 60 250 25th 0.454 30th 1.26 600 60 250 15th 0.315 20th 1.26 350 65 150 20th 0.204 20th

As shown in Table 2, each of the sensor elements had 10 has the leading end thickness index value which satisfies Inequality (2) and has a water resistance of 6 µl or more.

7th Fig. 13 is a graph of the result of evaluation of water resistance according to Example 2 shown in Table 2 against the leading end thickness index value along with the result of evaluation according to Example 1. As in 6th is off 7th It can be seen that the leading end thickness index value has a positive correlation with water resistance regardless of the element base size difference 1 . This suggests that good water resistance is ensured as long as each layer is designed so that the leading end thickness index value satisfies inequality (2) even in a case where the leading end protective layer 2 the laminated structure has a plurality of unit layers.

In addition, from Table 2 and 7th confirmed that from the sensor elements 10 of the example 2 each of the sensor elements 10 with the leading end thickness index value satisfying Inequality (3) has good water resistance of more than 10 µl, and further, each of the sensor elements 10 with the leading end thickness index value satisfying Inequality (4), has extremely good water resistance of more than 20 µl.

QUOTES INCLUDED IN THE DESCRIPTION

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Patent literature cited

• JP 201665852 [0002, 0004]

Claims (6)

A sensor element for a gas sensor, the sensor element comprising: an element base that is a ceramic structure, the element base having an end surface with a gas inlet through which a measurement gas is introduced into the element base, and a sensor part for detecting a gas component to be measured, and a Includes heaters for heating the sensor element; and a leading end protective layer which is disposed around an outer periphery of the element base in a predetermined area including at least the end surface and which is a porous layer including one or more unit layers, the leading end protective layer being provided on the end surface , around $$\frac{1}{\mathrm{100,000}}{\displaystyle \sum \frac{{\text{T}}_{\text{j}}\cdot {\mathrm{\rho }}_{\text{j}}}{{\text{L.}}_{\text{e}}}>0.05}\left(\text{j}=1\sim \text{n}\right)$$

where T _j (j = 1 to n: n is a natural number) is a thickness in µm of a j-th unit layer of the leading end protective layer from one side of the element base on the end surface, p _{j is} a porosity in% of j -th unit _{layer and L e is} a distance in mm from the heater to the end surface. Sensor element for the gas sensor after Claim 1 wherein the sensor element has an elongated planar shape, and the end surface is a surface on a side of a leading end portion in a longitudinal direction of the sensor element. Sensor element for the gas sensor after Claim 1 or 2 , where n = 2, and the leading end protective layer is provided on the end surface to $$\frac{1}{\mathrm{100,000}}{\displaystyle \sum \frac{{\text{T}}_{\text{j}}\cdot {\mathrm{\rho }}_{\text{j}}}{{\text{L.}}_{\text{e}}}>0.18}\left(\text{j}=1.2\right)$$

to meet. Sensor element for the gas sensor after Claim 3 , where 300 µm T ₁ 850 µm, 40% p ₁ 80%, 150 µm T ₂ 350 µm, 15% p ₂ 40%, and 0.35 mm L _e 1.3 mm. Sensor element for the gas sensor after Claim 4 , wherein the leading end protective layer is provided on the end surface to $$\frac{1}{\mathrm{100,000}}{\displaystyle \sum \frac{{\text{T}}_{\text{j}}\cdot {\mathrm{\rho }}_{\text{j}}}{{\text{L.}}_{\text{e}}}>0.25}\left(\text{j}=1.2\right)$$

to meet. Sensor element for the gas sensor after Claim 5 , where 300 µm T ₁ 850 µm, 50% p ₁ 80%, 250 µm T ₂ 350 µm, 15% p ₂ 40%, and 0.35 mm L _e 1.3 mm.

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